1. Field of the Invention
This invention pertains to oscillator circuits, and more particularly to self-excited oscillators.
2. Prior Art
Oscillator circuits typically employ one or more switching devices, as for example active semiconductor devices such as bipolar transistors, FETS, etc. Through a variety of possible arrangements, the semiconductor devices are alternately driven between increasing and decreasing conduction producing an oscillating output which is applied to a load, typically via the secondary winding of a transformer incorporated in the oscillator. One such configuration is the well known self-excited push-pull oscillator.
A principal problem with such circuits is transformer core saturation which, if left unchecked, can result in excess heating and reduced efficiency. Therefore, in a self-excited push-pull oscillator, an important design consideration is to effect reversal prior to core saturation. While the prior art has developed such schemes, they typically rely on one or more auxiliary magnetic devices, such as feedback transformers, driven to saturation for effecting reversal of the conductive states of the switching devices before saturation of the main transformer core occurs. This not only adds cost and complexity, but many such circuits suffer from a phenomenon known as "core walking". For example, in the case of a bipolar self-excited push-pull oscillator wherein the saturation voltage of the transistors is not identical, which is almost inevitably the case, with each succeeding cycle the collector current of one transistor will increase relative to the collector current of the other. Since core flux is proportional to current flow through the winding, the core flux will also increase, with each operating cycle, in the direction of the transistor with the higher collector current. Eventually, the core "walks" to saturation in the direction of that transistor.
It is accordingly an object of the present invention to provide a self-excited oscillator incorporating improved means for effecting reversal prior to core saturation.
It is a further object of the present invention to provide a self-excited push-pull oscillator wherein core walking is avoided.
It is another object of the present invention to provide a self-excited oscillator which avoids core saturation without the need for complex or costly circuit components, such as auxiliary magnetic devices.